Vertical conveyors are employed in warehouses, factories and the like to convey material or cargo between different vertical levels. The typical vertical conveyor includes a supporting structure or frame and a carriage, which is adapted to support cargo, is guided for vertical movement on the supporting structure.
In one type of vertical conveyor, the carriage is either straddled between two vertical columns of the frame or is cantilevered outward from the columns and is guided for vertical movement on the columns. The lifting of the carriage is accomplished through the use of two hydraulic cylinder units, each of which is mounted on one of the vertical columns. The piston rod of each cylinder unit is, in turn, connected to a flexible lift means consisting of either wire rope or roller chain, having one end connected to the carriage and the other end dead headed. A vertical conveyor of this type is described in U.S. Pat. No. 5,205,379. As described in the patent, each cylinder unit is affixed to the lower portion of its respective vertical column. The piston rod of each unit carries either a sheave adapted for wire rope or a sprocket adapted for roller chain. As the cylinder rod retracts, it pulls on either the roller chain or wire rope causing the carriage to elevate from the lower to the upper level. In typical designs, the wire rope is trained over four sheaves creating a four-part line so that each inch of piston rod travel produces four inches of carriage travel. In designs using roller chain, the chain is trained over two sprockets creating a two to one ratio of carriage movement to piston rod movement.
Other typical vertical conveyors use a pair of hydraulic cylinder units to elevate the carriage between levels. The cylinders are attached to each other and disposed so that one pistron rod pushes downward and one piston rod pushes upward against the carriage structure. With this design, care must be taken in design of the piston rod as the forces imposed by the weight of the carriage and the weight of the load being elevated impose a buckling force on the piston rods. As the cylinders are tied together, the piston rods are laterally offset from each other which results in a couple which puts considerable stress on the piston seals and packing flanges of the cylinder units. Because of the buckling and lateral forces on the cylinder units, this design is usually limited in capacity and vertical travel.